Individual mouse VL30 elements transferred to rat cells by viral pseudotypes retain their responsiveness to activators of protein kinase C

Complex interactions with direct repeats of a mitogen-responsive VL30 enhancer

The RVL-3 VL30 enhancer is an LTR-derived triple direct repeat of 35 base pairs that mediates gene induction in response to several different intracellular signaling pathways. Using mobility shift assays, methylation interference and DNase I footprinting, we have investigated the physical interactions between the RVL-3 enhancer and components of nuclear extracts from Rat-1 cells. Each enhancer repeat unit contains a single binding site. Our studies suggest that binding to the double or triple repeat enhancer is cooperative, involving simultaneous occupation of two sites, with a preference for adjacent sites. Binding cooperativity would have implications for the mechanism of gene activation directed from the native VL30 enhancer.

Anoxia, wound healing, VL30 elements, and the molecular basis of malignant conversion

Although VL30 retrotransposable elements have been associated with certain cancers for nearly twenty years, because of their expression in rodent malignancies and recombination into murine sarcoma viruses, their causative role, if any, in cancer has been uncertain and enigmatic. Recent findings suggest loss of normal transcriptional control of specific VL30 element expression may make a critical contribution to tumor progression at a step associated with malignant conversion, by bringing into play a cellular program normally involved in wound healing. This program, the fibroblast anoxic response system, includes an adaptation to glycolytic metabolism, secretion of metalloproteinases, and activation of an endonuclease. While appropriate for facilitating debris removal during wound healing, loss of control of this program in a cell which has already progressed to the benign neoplastic state has the potential to simultaneously produce the invasiveness and genomic instability characteristic of malignancy. Examination of tumors and tumor derived cell lines has confirmed that key aspects of this system are in fact activated in cancer.

Regulation of transin/stromelysin and VL30 gene expression by intracellular calcium

Changes in intracellular Ca++ levels are observed as a second messenger in response to a number of cellular agonists, including epidermal growth factor, transforming growth factor beta 1, and endothelin-1. The role of elevated intracellular Ca++ in transducing the effects of these three agonists on gene expression has been studied using two target genes: transin/stromelysin-1 and the endogenous murine retrovirus VL30. Although the effects of EGF and TGF beta 1 on transin/stromelysin-1 mRNA expression appear to be independent of these agonists' effects on intracellular Ca++ levels, elevated Ca++ interacted synergistically with activators of pkC to induce transin expression, even though neither agent alone could induce transin/stromelysin-1 expression. In contrast, the integrated VL30 retrovirus could be induced by Ca++ ionophores alone, and induction of VL30 mRNA by other agonists was blocked if intracellular Ca++ levels were held below a threshold value of 165 nM with Ca++ chelators. Genetic analysis of the VL30 upstream regulatory region indicated that a triple-repeat element present in the VL30 long-terminal repeat could function as an inducible enhancer, but responsiveness to either EGF or pkC activation required the concomitant elevation of intracellular Ca++. Because EGF was capable of inducing expression even in pkC-depleted cells, providing Ca++ levels were elevated, these results indicate that elevated intracellular Ca++ is capable of interacting synergistically with multiple signaling pathways to stimulate increased gene expression.

Specificity of retroviral RNA packaging

Encapsidation of retroviral RNA has been shown to be dependent on specific cis-acting signals, in particular, the packaging region (psi) located near the 5' end of the retroviral genome. In this report, we show that a 683-base avian extended packaging sequence (psi+) derived from Rous sarcoma virus will direct packaging of heterologous hygromycin mRNA into avian virions when present at the 3' end of the transcript in the sense orientation. However, this packaging is not as efficient as the packaging of RNA encoded by a standard avian retroviral vector. A quail cell line containing a Rous sarcoma virus mutant, SE21Q1b, produces virions which will package endogenous cellular mRNAs randomly, roughly in proportion to their intracellular concentrations. We found that viral particles from SE21Q1b retain the capacity to specifically encapsidate hygromycin mRNAs containing the avian psi+. To determine whether packaging of cellular mRNA would occur in other retroviral packaging lines, we assayed virion RNA isolated from the retroviral particles produced by avian and murine packaging lines for the presence of endogenous cellular mRNAs. Endogenous cellular mRNAs were not found randomly packaged into virions produced by any of the packaging lines examined except SE21Q1b. Some specific sequences, however, were found packaged into avian virions. Endogenous retrovirus-related mink cell focus-inducing murine leukemia virus RNAs and 30S viruslike RNAs were found to be efficiently packaged into murine virions even in the presence of RNAs containing all cis-acting retroviral sequences.

Retrotransposon-like VL30 elements are efficiently induced in anoxic rat fibroblasts

VL30 elements are a multigene family within the class of retroviruses and retrotransposons. We have characterized the response of normal rat fibroblasts to anoxia, in which endogenous VL30 element expression is strongly induced. Optimal induction up to 500-fold occurs under complete anoxia, although a lesser response is seen under atmospheres up to 2% oxygen. Phorbol esters and diacylglycerol, which induce mouse VL30 RNA approximately eightfold, show no effect on the rat VL30 system. The hypoxic conditions optimal for rat VL30 induction represent a mild cellular stress, with no reduction in cell viability during the induction period. Although the precise physiological role of this fibroblast response to temporary anoxia is unknown, it may occur during wound healing. The induction of VL30 by anoxia provides a unique model system wherein a member of the mammalian retrovirus/retrotransposon family is highly responsive to a common physiological signal.